1. Field of the Invention
This invention relates to a demodulation device of a 2.sup.n -value (n=1, 2, 3, . . . ) quadrature amplitude modulation type digital radio communication device and more particularly to the improvement of an abnormal synchronization preventing circuit in the demodulation device. In the 2.sup.n -value quadrature amplitude modulation type digital radio communication device of this invention, data of high transmission speed can be processed even if a device of low operation speed is used.
2. Description of the Related Art
Recently, with the increasing needs for desired communication and with the development of the communication technology, various types of communication systems have been developed. Among them, a digital microwave radio communication system is included.
The digital microwave radio communication system modulates a carrier wave of microwave, for example, by use of the quadrature phase shift keying (QPSK) system or multiple value quadrature amplitude modulation (multiple value QAM) system and then transmits digital data by radio.
The QAM system which is a typical quadrature modulation system is a system for changing both of the amplitude and phase of the carrier wave and a highly efficient modulation can be attained by thus changing the two parameters simultaneously. The QAM system can be effected in principle by combining two quadratic AM-modulated (amplitude-modulated) waves and, as a result, the thus derived signal is called a QAM wave.
Since the QAM wave is created based on an AM wave which is easiest to deal with and a desired point on the phase plane can be selected as a coding point, an ideal code arrangement can be attained and the QAM wave may play an important role in the multiple value transmission.
The QAM is divided into a QAM for converting the quantized values of the two quadrature AM waves into binary values and a QAM for converting the quantized values into multiple values, and the latter QAM is called a multiple value QAM.
Now, the principle of the QAM is explained. The feature of QAM is effectively used in the multiple value transmission of 16 or more values, but in this case, a 4-value transmission is used as an example in order to simplify the explanation.
Like the analog modulation system, the basic modulation system for modulating a sine carrier wave by use of a digital signal is divided into three modulation systems: amplitude modulation system, phase modulation system and frequency modulation system. In the digital modulation, the above modulation systems are also called as follows. That is, the amplitude modulation system is an ASK (amplitude shift keying) system, the phase modulation system is a PSK (phase shift keying) system and the frequency modulation system is a FSK (frequency shift keying) system.
In general, a system for transmitting a signal by use of n coding points arranged at a regular interval (2.pi./n) on a circumference indicating the phase of the carrier wave is called an n-phase PSK, but in an actual case, n is set to 2.sup.m (m is a natural number), and in this case, transmission of binary pulse m series can be effected.
A 4-phase PSK wave can be derived by a quadrature combination of two 2-phase PSK (phase shift keying) waves and the 2-phase PSK wave can be derived by use of a binary ASK wave. Therefore, if the 4-phase PSK signal is E(t), two 2-phase PSK signals are e.sub.1 (t) and e.sub.2 (t) and the angular frequency is .omega.c, then the 4-phase PSK signal can be expressed by the following equation (1). ##EQU1## where .psi..sub.1 (t) and .psi..sub.2 (t) indicate waveforms of independent binary base band signals (modulated signals) and are defined as follows. EQU .vertline..psi..sub.1 (t).vertline..ltoreq.1, .vertline..psi..sub.2 (t).vertline..ltoreq.1 (2)
Further, E(t) can be expressed as follows by use of the combined amplitude (envelope) and phase angle. ##EQU2## Since the bandwidths of .psi..sub.1 (t) and .psi..sub.2 (t) are not infinite and are generally limited, the amplitude thereof becomes smaller than "1" at time other than the center of the pulse (sampling time) and the absolute value of E(t) becomes smaller than "1". This indicates that the locus of E(t) is drawn along a square ABCD and diagonal lines AC and BD in the vector diagram of FIG. 13. In this case, E(t) is different from a pure PM wave, but since it satisfies the condition of constant amplitude when the range is limited to the sampling time, it can be regarded as a PSK wave. Like E(t), a signal which can be derived by a combination of two quadrature AM waves is the QAM wave.
FIG. 14A indicates the code arrangement on a 2-dimensional phase plane in the case of 16 values and indicates a grid-form QAM. FIG. 14B is a diagram indicating PSK.
The grid-form QAM as shown in FIG. 14A has a relatively good SN characteristic and the quadrature modulation technique for superposing information on the sine and cosine components of the carrier wave can be used. The grid-form QAM can be derived by combining two quadrature AM signal waves of n values (generally, n=2.sup.m) and has n.sup.2 coding points.
Assuming now that .psi..sub.1 (t) and .psi..sub.2 (t) are base band pulses (modulated signal) having amplitudes of n values and the maximum values of the absolute values of .psi..sub.1 (t) and .psi..sub.2 (t) are set to "1", the general equation of the waveform E(t) of unit amplitude becomes equal to the equation (1). If E(t) has 2.sup.2m coding points, four coding points are set at the same distance from the original point when m=1, and the code arrangement coincides with the code arrangement of the 4-phase PSK. It is called 16 QAM, 64 QAM and 256 QAM when m=2, 3 and 4, respectively.
The QAM wave of 16 or more values satisfies the condition that both of the amplitude and phase contain information. Therefore, the 16 or more QAM may be used as the QAM wave and the modulation method most widely used in the recent digital radio system is the 16 QAM.
When a digital radio signal which is modulated by use of the multiple value QAM is reproduced, the demodulation must be effected by use of synchronous detection since both of the amplitude and phase contain information.
A case wherein the 16 QAM is used is explained. In the reception system, a received carrier wave is divided into two portions which are subjected to the synchronous detection by use of two reference carrier waves of 90.degree. phase difference (since QAM is derived by a combination of two quadrature AM waves, a reference carrier wave having a phase of one of the two axes which intersect at right angles, for example, I axis and a reference carrier wave having a phase of the other axis or Q axis are used). By the synchronous detection, detected outputs of the I-axis component and Q-axis component can be derived.
For the detected outputs of the I-axis component and Q-axis component, that one of the 16-level codes which is received is determined by an identifying circuit and 4-series binary pulses are reproduced according to the type of the received code. That is, in the case of 16 QAM wave, one of the 16-level codes is determined by the identifying circuit, converted into 4-bit parallel data and output.
In order to create the reference carrier wave for determining the phase reference at the time of synchronous detection, the following methods may be used as a typical method. One of them is a method of deriving a reference carrier wave by using an independent carrier generator (generally, a voltage-controlled oscillator VCO is used) provided in the receiver and controlling the phase of the carrier generator to a constant value, and another method is a method of extracting part of the received signal, delaying the extracted part by time of one time slot and using the delayed signal as a reference carrier wave for the succeeding pulses. The latter method is a method used for delayed detection and therefore the former method is generally used.
In the PSK synchronous detection method, the phase reference must be derived from the received signal transmitted. However, in this case, since there occurs a problem that the phase of the received signal of PSK continuously varies by modulation, a constant control signal must be created by removing the modulation component and fed back to the voltage-controlled oscillator in order to derive the reference phase. The frequency multiplying method or inverse modulation method may be used as a method for removing the influence due to the modulated component.
The frequency multiplying method is called a base band processing type method since the demodulation signal of the base band bandwidth is multiplied. As a base band processing type carrier reproducing circuit, a circuit for effecting the logical operation for the demodulation signal in the base band bandwidth, effecting the equivalent operation of multiplying the phase and controlling the phase of the reference oscillator according to the output thus obtained and it is also called a Costas type carrier reproducing circuit.
When a Costas type carrier reproducing circuit is used, the circuit construction thereof may be made simple at a low cost in comparison with a case wherein the inverse modulation method is used and the circuit is advantageous in the economical point of view and in the low power consumption and therefore it is widely used.
Thus, the Costas type carrier reproducing circuit has the above advantages, but the operation thereof becomes stable at a frequency different from a normal carrier frequency in the case of low speed transmission and a problem of abnormal synchronization in which the carrier synchronization becomes abnormal, for example, may occur. If the carrier reproducing loop characteristic is changed in order to solve the above problem, another problem that the pull-in frequency and complex modulation characteristics are deteriorated may occur.
Further, the data transmission speed generally tends to be enhanced, but a circuit of high-speed operation must be used as a circuit device in order to cope with the high speed transmission. However, the cost of the device of high speed operation is high, and in order to cope with the high speed transmission, the system price will be significantly enhanced. Therefore, a method of reducing the cost of a system of high-speed data transmission is an important subject.